A nozzle box assembly for a steam turbine generally includes three components: a torus, a bridge ring, and a steam path ring. Each of the components is initially formed in 180° segments, followed by welding the components together to form two nozzle box halves. The halves are then joined together along a horizontal midline to form a steam box assembly for a steam turbine. Each nozzle box half includes one or more steam inlets formed integrally with the torus. These inlets extend from the torus in a plane normal to the axis of rotation of the turbine. During steam turbine operation, the inlets receive steam from a suitable source for flow into the torus. The steam changes direction to a generally axial flow for flow through the annular opening of the bridge ring and into a steam path ring having a series of nozzles which include airfoil vanes for directing the steam flow to subsequent buckets.
Transitions between the torus, bridge ring, and steam path ring along the steam path side disturb the flow of steam from the turbine main steam inlets. This tends to cause turbulence in the steam flow from the main steam inlets as it passes through the bridge ring into the steam path ring, which then causes a loss of efficiency. Reducing the turbulence in the steam path would allow for optimized flow through the nozzle box and increased efficiency of the steam turbine.